Field of Radium B on Recoil from Radium A. 885 



in a manner similar to that adopted by Rutherford in his 

 experiment on the magnetic deflexion of the a-particles *. 



The distance between the bands was in this way found to 

 be 2'86 mm. Since d 1 = o'6 and tf 2 = 4*85 we have as above 



P =61, 



and since the field was 10,700 gauss, we have 



— = B.p = 6'52xl0 5 . 



e 



This value is certainly more reliable than that obtained by 

 the first method described above, and is in fair agreement 

 with it. 



It is of interest to compare the value of Hp obtained with 

 that to be theoretically expected. Now the momentum of 

 the recoil-atom of radium B which is produced on the 

 emission of an a-particle from radium A must be the same as 

 the momentum of this a-particle, since an atom of radium A 

 gives out only one a-particle when it is transformed into 

 radium B. It therefore follows that if the charge carried by 

 radium B on recoil is the same as that carried by an a- 

 particle, the value of Up for the radium B should be the 

 same as for the a-particle from radium A. Now it has 

 been shown by Rutherford f that the value of HjO of this 

 a-particle is 3*48 x 10 5 or nearly half that for the recoil-atom 

 of radium B. It therefore appears that the charge carried 

 by the radium B is half that on an a-particle, or, in other 

 words, the atom of radium B carries with it the same charge 

 as the hydrogen ion in electrolysis ; for it is known that the 

 a-particle is associated with twice that charge J. 



Although the experimental values so far obtained are not 

 of very great accuracy, it is possible to calculate from the 

 electric and magnetic deflexions of radium B the velocity of 



e 

 these particles and the value of — . Since the charge carried 



L . m ° 



by the particles has been shown to be the same as that carried 

 by the hydrogen ion in electrolysis, a knowledge of the latter 

 quantity gives the atomic weight of radium B. 



Taking the path of the particles in an electric field as 

 approximately circular, from the experiments of Russ and 

 Makower we have that with a field of 16,250 volts per 



* Rutherford, Phil. Mag. Aug. 1906. 

 t Rutherford, Phil. Mag. Aug. 1906. 

 \ Rutherford and Ueiger, Proc. Roy. Soc. A., vol. lxxxi. 1908. 



